Undersize a supply and it sags, overheats or shuts down on a transient you never measured. Sizing it is mostly about being honest with yourself about the peak, not the average.
Add up the rails properly
List every load per voltage rail with its worst-case current, not its typical. Motors, solenoids and radios draw far more when starting or transmitting than when idling. Sum each rail, then add 20–30% headroom — for capacity you will inevitably add, and because supplies run hotter and age faster near their limit.
Inrush is the thing that trips breakers
Capacitive loads and motors pull an enormous spike at switch-on — many times the running current, for a few milliseconds. That is what nuisance-trips a breaker or welds a relay contact. Limit it with an NTC thermistor, a soft-start, or a supply with a controlled ramp. Size fuses for the I²t let-through, not just the steady current (see the fuse tool).
A 90%-efficient supply delivers 100 W to the load. How much heat does it put into the enclosure?
Efficiency is output over input, so the input is 100/0.9 = 111 W and the loss is the difference: 11.1 W, not 10. The mistake is small at 90% and gets ugly fast — at 70% efficiency a 100 W load makes 43 W of heat, not 30. Worth getting right, because that figure is the one your enclosure and its free-air temperature rise have to cope with, and it is the reason a supply that is comfortable on the bench cooks in a box.
Try it — load, headroom and heat
Efficiency becomes heat
A 90%-efficient 100 W supply dumps ~11 W into your enclosure. A linear regulator dropping 12 V to 5 V at 1 A wastes 7 W as heat — check it with the regulator tool, and switch to a buck converter if that number is uncomfortable. Then make sure the enclosure can actually shed it (heat-sink tool).
Don't forget
Check the minimum load — some switchers misbehave unloaded. Confirm hold-up time if a brownout must not reset the controller. Derate hard for high ambient temperature. And verify the supply's own connector and wiring ratings carry the current you just specified.